The Collaborative International Dictionary of English v.0.48:

Encroacher \En*croach"er\, n.
   One who by gradual steps enters on, and takes possession of,
   what is not his own.
   [1913 Webster]

WordNet (r) 3.0 (2006):

encroacher
    n 1: someone who enters by force in order to conquer [syn:
         invader, encroacher]